Elongated hollow members formed by extruding aluminum or other light-weight metals into a prescribed cross sectional shape have lately been drawing attention as automotive frame members because they can be freely given with a desired cross section and may be different in thickness in two different directions so that a desired mechanical strength can be achieved for each individual part of the vehicle body in a highly efficient manner.
In a so-called space frame formed by welding metallic tube or other shaped members, the rear pillar is sometimes curved in such a manner that an intermediate part of the rear pillar bulges out laterally outward to maximize the opening of the tail gate and a lower end of the rear pillar is curved inward to avoid interference with a rear wheel.
When a rear pillar given with such a shape is subjected to a bending load or a twisting load, a relatively large stress tends to develop in the parts having a relatively large curvature while no such large stress tends to develop in parts given with a smaller curvature. In particular, the mechanical stress tends to concentrate in the part of the rear pillar located in an upper part of the bulge for accommodating the rear wheel. Therefore, to optimize the overall rigidity of the frame, it was necessary to weld reinforcing members to the parts where the stress tends to concentrate or to increase the wall thickness of such parts. However, such measures inevitably led to an increase in the weight, and compromised the advantage of the space frame to allow the weight to be reduced.